Nonlinear optical properties of materials

This research project is executed by Willem Beeker

Daily Supervisor: Guus Rijnders

Project in collaboration with the group Laser Physics of Prof. Klaus Boller

The way light acts upon matter can be described by a dipole of an electron-proton pair that is proportional to the Electric field strength P = chi*E. This linear response is only valid for small perturbations of the electrons in a material. For stronger perturbations (a stronger light field) this dipole relation may have nonlinear terms. The second chi term which is then multiplied by E^2 will only have a reasonable size in non-centrosymmetric crystal structures or, for instance, at boundaries. Materials with this property are used for frequency doubling of lasers, or can be used for their electro-optic effects. The third chi term plays a role in very strong fields and leads to a refractive index that depends on the intensity of the light. This is known as the nonlinear Kerr effect. This property is present in all crystal structures.

Part of the research will be the characterization of the strength of the nonlinear effect in either bulk or thin films. Thin layers of these nonlinear optical materials can be used in chip based devices as the optical active part for all optical switching for instance. As these devices will also consist of optical waveguides the interaction between such waveguides and the nonlinear thin films will be studied.

Light conversion processes such as second harmonic generation or sum frequency generation are increased under specific circumstances named phase matching. This phasematching may be due to birefringence or by poling a material, where the poling length is critical. Random phasematching will be studied as a means to increase the effective nonlinear effect. Another approach will be to study a structured growth of the nonlinear material on a substrate.